This invention relates to means for packaging perishable food. In particular, this invention relates to means for packaging perishable food in a modified atmosphere package. More particularly, this invention relates to packaging food with a modified atmosphere package having at least one film layer comprising a blend of a homogeneous linear or substantially linear ethylene polymer and a polypropylene homopolymer or copolymer.
In the modern distribution and marketing of food products, a multitude of differer packaging materials are used. One principal category of food packaging materials is plastic film. Many kinds of plastic film exist, which differ both in composition and structure, with some being tailored to specific applications and with others are more generic in nature.
Different types of produce packaging are illustrated in the following examples. Bulk carrots sold in bunches directly from a shipping crate are considered to be "unpackaged" although it is recognized that they must be contained in some type of box crate for shipment. Lettuce that is loosely wrapped in a protective film would be considered to be to be minimally packaged because, although some degree of protection provided by the wrap, the package can breathe freely and the lettuce can be contaminated fairly easily. A mixture of cleaned and ready-to-eat iceberg lettuce, carrots, and cabbage in a sealed bag is an example of fresh-cut produce contained in a modified atmosphere package.
Modified atmosphere packaging systems are packaging systems which maintain an environment surrounding the perishable item which slows spoilage. Many bulkproduce items that have historically been shipped and sold unpackaged or minimally packaged can benefit from proper containment in a modified atmosphere package. Modified atmosphere packaging films serve to extend shelf-life, and thus reduce the amount of discarded produce, improve quality by slowing produce aging and by reducing exposure to bacteria, and promoting convenience to the consumer due to the availability of high quality pre-cut produce.
Modified atmosphere packaging works to extend the life of fresh-cut produce by reducing the respiration rate and associated aging of the produce. After produce is picked it continues to live and breathe, or respire. During this time the produce consumes oxygen and gives off carbon dioxide. This is the opposite of photosynthesis, during which plants consume carbon dioxide and give off oxygen. One reaction, of many, that occurs during the respiration process is the conversion of glucose and oxygen to water and carbon dioxide: ##STR1##
To reduce the respiration rate of the produce, one can reduce the concentration of oxygen in the package and/or increase the concentration of carbon dioxide in the package When the fresh-cut produce is exposed to an environment in which the oxygen concentration has been reduced, the respiration and aging of the produce is also reduced. This extends the usable shelf-life of the produce and improves the quality of the produce. However, while the oxygen concentration in the package should be reduced, it should not be eliminated, as such would lead to anaerobic respiration and rapid spoilage. For this reason, high barrier packages, which prevent most transmission of oxygen and other gases are generally not suitable for long term packaging of living fresh-cut produce. Packages designed with selective barrier properties that effectively control oxygen transmission rates and the resulting oxygen concentration inside the package are the essence of modified atmosphere packaging used for fresh-cut produce
Increased carbon dioxide concentration may also reduce the respiration rate of the produce. For certain foods carbon dioxide also inhibits the growth of certain microorganisms. Carbon dioxide acts as a fungicide for strawberries, for example. Some types of produce are sensitive to high concentrations of carbon dioxide, however. For example, iceberg lettuce may discolor if the carbon dioxide concentration exceeds about 2.5%.
In addition to oxygen and carbon dioxide concentration, there are many other factors that determine the rate at which the produce respires, e.g., temperature, the age an condition of the produce, water content, and ethylene concentration in the environment. In the case of temperature, many types of produce are stored at or below 40.degree. F. to slow respiration and therefore slow aging. However, care must be taken not to expose the produce to temperatures that are below the temperature at which the produce will undergo irreversible damage.
In addition to food preservation properties, film and package fabricators, as well as the ultimate consumers, impose additional requirements. From the perspective of the fabricator, the packaging film must have the physical properties necessary for good machinability during the packaging process and good package integrity during distribution and display to prevent disruption of the modified atmosphere. A key property for good machinability is sufficient stiffness or modulus. Key properties for good package integrity are good heat seal performance and high tear, puncture, and impact resistance.
From the perspective of the consumer, exceptionally good optical properties in a packaging film are essential to enable the consumer to visually inspect packaged produce before purchasing it. Further, higher modulus films are easier to fabricate into packages and have greater appeal to the consumer than flimsy, soft packages.
While modified atmosphere packages are currently employed in the packaging of fresh-cut produce, none exhibits an optimal balance of the requisite performance attributes. Common plastics (e.g., oriented polypropylene, polystyrene, polyester), which meet optics and modulus criteria, have poor oxygen permeability rates, heat seal performance, and tear resistance. Other plastics which meet optics, permeability, and heat seal performance requirements, such as 18% VA ethylene vinyl acetate copolymers have poor machinability and low tear resistance. Still other plastics which meet permeability rate, machinability, heat seal and tear resistance have insufficient optical properties (e.g., heterogeneous linear ethylene polymers, often referred to as "ULDPE" and "VLDPE").
In this regard, U.S. Pat. No. 5,139,855 discloses a stretch wrap which comprises a core layer and EVA skin layers. The core layer is a blend of 5-30 wt. % polypropylene and 70-95 wt. % of what the patent refers to as "VLDPE". U.S. Pat. No. 5,139,855 does not purport to obtain oxygen transmissive films. Moreover, the films disclosed lack the excellent optical properties which are critical for packaging fresh-cut produce and other perishable foods.
U.S. Pat. No. 5,389,448 discloses a multilayer packaging film having improved burn-through resistance, which is attributable to a film layer comprising from 20-80 weight percent polypropylene (preferably 40 or 60 weight percent polypropylene) and 80-20 weight percent of what the patent refers to as "VLDPE". Preferably, the VLDPE will have an I.sub.2 of no more than 0.15 g/10 min. One preferred polypropylene is described as having a melt flow rate (ASTM D 1238, Condition 230/2.16) of from 0.6 to 0.8 g/10 min. Again, the films disclosed lack the excellent optical properties which are critical for packaging fresh-cut produce and other perishable foods.
High modulus polymers, such as polypropylene and styrene-butadiene copolymers provide stiffness, but even as thin layers in coextrusions they do not provide the high transmission rates obtainable with films made of polyolefin resins, such as high percent vinyl acetate ethylene-vinyl acetate copolymers (EVA) or homogeneous linear or substantially linear ethylene polymers. These high-stiffness polymers also have low tear resistance and poor sealability.
To improve machinability and heat seal performance, designers have used coextrusions which often decrease permeability rates of the film, as the lower permeability layer serves as a barrier. To increase permeability, designers have sometimes perforated films of high modulus materials. However, perforated films do not have sufficient selectivity to distinguish between oxygen and carbon dioxide molecules. Accordingly, perforated films sacrifice the excellent carbon dioxide/oxygen transmission ratio available with nonperforated films, since oxygen and carbon dioxide molecules transmit through the holes at closer to equal rates. Moreover, perforated films raise concerns regarding sanitation. An additional drawback to such processes is that the fabrication technology for coextruded films and/or laminated films and for perforation technology is relatively sophisticated, increasing the capital investment required to enter the film fabrication business.
In many fresh cut produce packaging applications, films must meet minimum requirements in terms of optics, sealability, modulus and abuse resistance. Additionally oxygen transmission requirements must be met which vary for different types of produce. With current technologies, lower-than-desired-modulus films are often used to achieve critical transmission rates for high respiring produce, and lower modulus means poorer performance on form/fill/seal equipment and poorer consumer appeal. Unless costly and often undesirable perforation techniques are used, current technologies provide a limited level of oxygen transmission at any given modulus and a limited modulus at any given oxygen transmission rate. For instance, using currently available technology, to obtain oxygen transmission rates greater than 1000 cc(at STP)-mil/100 sq.in.-day requires the use of film having a modulus which is insufficient to be preferred for use in vertical form/fill/seal applications (unless often undesirable perforation techniques are employed) Such low modulus films are disadvantageous for use in vertical form/fill/seal machines, a they tend to bunch up around the forming collar or other parts of such machines, resulting in deformed or improperly sealed packages. Such low modulus films have limited consumer appeal, due to their flimsy nature.
Industry would find great advantage in a modified atmosphere package which exhibits good oxygen transmission, high modulus, and good optical properties. Industry would find particularly great advantage in a modified atmosphere package which further exhibits good heat seal performance and high tear, puncture, and impact resistance.